1. Field of the Invention
The present invention relates to a method of making a light emitting diode, in particular a GaP red light emitting diode (hereinafter referred to as GaP red LED) for attaining a superior forward characteristic and high intensity of light.
2. Prior Arts
A GaP red LED emits light by utilizing an exciton recombination phenomenon due to Zn-O pairs as nearest neighbor pairs inside a p-type epitaxial layer. Previously, such Zn-O pairs are produced in the GaP red LED after carrying out the following processing procedures:
(i) Zn and then O in a form of Ga.sub.2 O.sub.3 are doped in a Ga melt containing polycrystalline GaP,
(ii) this solution is heated at a temperature of 1,000.degree.-1,060.degree. C. thereby sufficiently heating the doped melt, then
(iii) the surface of an n-type GaP substrate is contacted with the melt for a predetermined period,
(iv) a p-type epitaxial layer containing Zn and O is grown on the n-type GaP substrate by cooling the system at a cooling rate of 3.degree. to 10.degree. C./min., and finally
(v) a heat treatment lasting for a specified time, e.g., 15 hours is performed on the n-type GaP substrate with a p-n junction at a low temperature around 600.degree. C.
FIG. 1 is a graph showing the abovementioned procedures with respect to a relationship between time duration and processing temperature. The treatment temperature is on the ordinate scale, while the time is on the abscissa. The conventional method of forming the GaP red LED comprises the steps of
producing the melt by heating at 1,000.degree.-1,060.degree. C. for about 60 minutes between t=t.sub.0 and t=t.sub.1, contacting the surface of the n-type GaP substrate with the melt at the same temperature for about 30 minutes between t=t.sub.1 and t=t.sub.2,
growing the p-type epitaxial layer containing Zn and O atoms by cooling down the n-type GaP substrate at a specified cooling rate of e.g. 3.degree.-10.degree. C./min. between t=t.sub.2 and t=t.sub.3,
carrying out further cool-down between t=t.sub.3 and t=t.sub.4 at a cooling rate of e.g. 10.degree. C./min., and then
carrying out the heat treatment at about 600.degree. C. for, e.g., 15 hours between t=t.sub.4 and t=t.sub.5 to form of the Zn-O pairs.
In order to increase the light intensity of the GaP red LED, it is necessary to increase the concentration of the Zn-O pairs acting as radiative recombination centers in the p-type epitaxial layer, to decrease the number of free holes serving as non-radiative recombination centers, and to improve an injection efficiency of the electrons into the p-type layer.
The Zn-O pairs consisting of Zn as an acceptor and O as a donor are electrically neutral. This means that the carrier density in the p-type epitaxially grown layer decreases after the heat treatment which is made for the purpose of the formation of the Zn-O pairs. The decrease of the carrier density has a tendency as shown in FIG. 2. That is, the carrier density gradually decreases from the inside part towards the surface part of the p-type epitaxial layer. The decrease of the carrier density makes it difficult to form a contact of good ohmic characteristics at the p-type epitaxial layer. In addition, due to the decrease the spreading resistance of the contact area and the series resistance of the p-type epitaxial layer increase, and accordingly the forward characteristic as one of the important characteristics for an LED is degraded.
In order to avoid such difficulties, several methods are employed. For example, p-type impurities are additionally diffused into the p-type epitaxial layer formed on the n-type GaP substrate thereby increasing the carrier density at the surface region. Alternatively, a p.sup.+ -type epitaxial layer having a large carrier density is further grown on the p-type epitaxial layer by use of Zn. After increasing the carrier concentration at the surface region, a contact region is formed thus making a GaP red LED. However, in the former method, the additional diffusion of the p-type impurities is carried out at a temperature much higher than the heat treatment temperature (around 600.degree. C.) for the formation of the Zn-O pairs, and accordingly the separation of the Zn-O pairs takes place. This separation is reflected by the difficulty the emission intensity is drastically reduced by the additional diffusion. On the other hand, in the latter method, an epitaxial growing process employing two kinds of solutions is required, thus remarkably lowering the processing efficiency.